1. Field of the Invention
The present invention relates to an optical disc recording and reproducing apparatus preferable for use with a writable optical discs such as a write once optical discs (CD-Rs) and a rewritable optical discs (CD-RWs).
2. Description of the Related Art
In recent years, writable optical discs such as write once optical discs (CD-R) and rewritable optical discs (CD-RW) standardized in ISO/IEC13490-1 have proliferated as information media for editing and recording audio data. In each of these optical discs 10, as shown in FIG. 1A, grooves 1 for guiding a light beam (hereinafter, referred to as pregrooves) are formed, and a positioning method referred to as a tracking servo is employed. The tracking servo is a mechanism where pits and projections constituting lands 2 formed on both sides of each pregroove 1 are detected to determine the position of an optical pickup, so that the desired pre-format address is accurately irradiated with a laser.
Sloped surfaces of the land, which coincide with side surfaces of the pregroove 1, are formed with a slight wobble in the form of a sine wave in-phase with each other as shown in FIG. 1B. The wobble signal indicates that the wobble component has been subjected to FM modulation. In the wobble signal, time axis information which indicates the position on the optical disc 10,
Sloped surfaces of the land, which coincide with side surfaces of the pregroove 1, are formed with a slight wobble in the form of a sine wave in-phase with each other as shown in FIG. 1B. The wobble signal indicates that the wobble component has been subjected to FM modulation. In the wobble signal, time axis information which indicates the position on the optical disc 10, and a recommended value of the power of the laser beam optimum for recording are encoded.
The encoded time axis information is referred to as ATIP (Absolute Time In Pregroove) information, and is written as an absolute time in the signal recording region (i.e. a program region) of the optical disc 10 along a direction from the starting point at its inner peripheral side toward its outer peripheral side. The ATIP information is written in the processing of the CD-R and CD-RW. The ATIP information is written in the optical disc 10 such as CD-R and CD-RW in a modulation mode referred to as bi-phase modulation mode. In this mode, a baseband modulation is conducted where, depending on whether the waveform of the last pulse signal constituting the error correction information of the immediately preceding pre-format address ends at a high level or low level, the pre-format address in the next frame is connected without being inverted, or the pre-format address is inverted and connected.
The ATIP information is written, for example, in the side surfaces Us, Ut of the pregroove 1 between two lands 2a and 2b shown in FIG. 1B. That is, the ATIP information is written in the side surface Us of the land 2a located at the inner peripheral side of the pregroove 1 in which data is recorded, and in the side surface Ut of the land 2b located at the outer peripheral side of the pregroove 1 in such a manner that the ATIP information written in the side surface Us is synchronous to the ATIP information written in the side surface Ut.
Therefore, the ATIP information can be read as a wobble signal by detecting the reflected light from the main spot 4 on the wobble formed in the lands 2a and 2b by two light receiving elements split in the direction of a track. The broken double line circles in FIG. 1B show spot diameters of the light beam applied to standard density optical disc.
The wobble signal is produced in such a manner that its center frequency becomes, for example, 22.05 kHz, when the optical disc 10 is rotated at a standard velocity (CLV) of a compact disc (CD). One sector of the ATIP information is constituted in such a manner as to coincide to one data sector after the signal is recorded. Therefore, at the time information is recorded, the pregrooves 1 are irradiated with a light beam with a predetermined intensity and information is written therein while the sector of the ATIP information is synchronous with the data sector.
In a conventional method, when a signal is recorded in the optical disc 10, as shown by the broken line in FIG. 1B a pit 61 is formed at a main spot 4 created by a three-beam method, and in this state, the return light from the main spot 4 is split and received in the photodetecting device 20 shown in FIG. 2. In the photodetecting device 20, a light receiving signal A from the light receiving element PD1 such as a photodiode which constitutes a four split photodetector, and a light receiving signal D from the light receiving element PD4 similar to the light receiving element PD1 are added to each other by a 2-input operational amplifier (OPA) 22A to produce an addition signal A+D. At the same time, a light receiving signal B from the light receiving element PD2, and a light receiving signal C from the light receiving element PD3 are added to each other by a operation circuit 22B to produce an addition signal B+C. In the photodetecting device 20, the latter addition signal B+C is subtracted from the former addition signal A+D by an operation circuit 22C and filtered by band pass filter 26 to obtain a wobble signal.
However, when data is written to the optical disc 10 by a laser beam, the large writing power of the laser beam received by the photodetecting device 20 causes the levels of the light receiving signals A.about.D to exceed the allowance value of the operation circuits 22A and 22B, and thus causing saturation of operation circuits 22A and 22B.
Therefore, a sampling and holding device 24 is provided between the photodetecting device 20 and the operation circuits 22A and 22B to turn off the connections between the photodetecting device 20 and the operation circuits 22A and 22B when data is written to the optical disc. By sampling and holding the signal between reading and writing data, the large writing power of the laser beam received by the photodetecting device is avoided.
However, when the sampling and holding device 24 turns off the connections between the photodetecting device 20 and the operation circuits 22A and 22B, the signal provided to the band pass filter 26 is temporarily terminated. Thus, quality of the wobble signal deteriorates.
FIG. 3A shows the waveform of the signal output by the operation circuit 22C shown in FIG. 2, and FIG. 3B shows the waveform of the wobble signal output by the band pass filter shown in FIG. 2. As shown in FIG. 3B, the amplitude of the wobble signal is irregular, which worsens the accuracy of the position on the optical disc indicated by the wobble signal.
In addition, the switching of the sampling and holding device 24 must meet the operation of the disc driver, thus complicating the timing setting of the switching of the sampling and holding device 24.